Deep understanding of relations between internal tablet structure and die wall pressure fluctuations in rotary tableting machines

Capping is a phenomenon in which cracks form at the tablet edges, leading to separation of the cap portion. Numerous studies have reported that residual die wall pressure, which is the die wall pressure after unloading, is a key factor in the occurrence of capping. This study investigated the relation between the internal structure of pharmaceutical tablets and residual die wall pressure during the tableting process. Using real-time die wall pressure monitoring and X-ray computed tomography imaging, we analyzed the correlation between internal crack formation and fluctuations in die wall pressure, particularly under conditions leading to capping defects. Two distinct residual pressure behaviors were observed, depending on the compaction force and powder type: a sudden decrease at lower forces (15 kN) and transient increase at higher forces (20 kN). Notably, tablets containing a mixture of acetaminophen and granulated lactose exhibited both increasing and decreasing pressure profiles, whereas lactose-only tablets showed a consistent increase at high compaction force. X-ray computed tomography analysis revealed that the vertical and circumferential distributions of cracks within the tablets strongly influenced the pressure fluctuation behaviors. Cracks located in the upper-edge region were associated with pressure drops, whereas those at both edges led to transient pressure increases. Moreover, die wall pressure fluctuations varied by strain gauge position, which reflected the circumferential crack distribution inside tablets. These findings suggest that die wall pressure measurement in a rotary tableting machine has the potential to predict the internal structure of a tablet.